|Basis: British Rail Class 04|
Mavis is a feisty diesel engine who is usually found working at the Quarry. She is full of her own ideas and doesn't take kindly to advice, which can sometimes get her into trouble. A friendly Diesel, Mavis is usually the first one Thomas turns to when the Steamies and Diesels need to work together.
- Mavis is based on a 0-6-0 Drewry diesel/mechanical locomotive.
- She has The Ffarquhar Quarry Co. Ltd. written on both her sides.